The present invention concerns a hybrid former for a paper machine, the former comprising a lower-wire loop which is jointly operative with the head box of the paper machine and which forms a first single-wire, preferably substantially horizontal dewatering zone in which water is removed from the web being formed by means of dewatering members placed inside the wire loop through the lower wire. Additionally, this former comprises an upper-wire unit which includes an upper-wire loop guided by guide and web-forming rolls, and forming, together with the run of the lower wire, a second twin-wire dewatering zone after the first dewatering zone. Furthermore, this former comprises a hollow-faced, first forming roll fitted inside the upper-wire loop at which the second, twin-wire dewatering zone begins and is curved upwardly over a certain sector of this forming roll. Additionally, a forming shoe is fitted inside the lower-wire loop after the forming roll and guides the second dewatering zone. The forming shoe is provided with a curved deck for guiding the lower-wire loop in particular, and has a center or centers of curvature on the side of the lower-wire loop.
It is known in the prior art that dewatering of a web that is being formed on a fourdrinier former of a fourdrinier paper machine takes place exclusively downwardly, whereby fines and fillers are removed from the web from the side of the wire due to the washing effect of either stationary dewatering members, e.g. foils, or revolving table rolls. This is the reason why a paper web produced by a fourdrinier machine is always anisotrophic with respect to properties of the two sides thereof, so that the upper face of the web is smoother and contains a larger amount of fines and fillers than the face that was situated against the wire which thus has a lower content of fines and fillers and in which, moreover, a so-called wire marking can be seen. Due to the above reasons, so-called twin-wire formers have been developed in which the forming of the web takes place for a significant part between two wires so that the two-sidedness of the web can be at least partially eliminated. Paper produced by means of such machines is also considered better, particularly with respect to printing qualities, than paper produced by means of fourdrinier machines.
In such prior-art twin-wire formers in which stationary dewatering members are not used, the formation is usually poor since it is impossible to produce pulsation of dewatering pressure which is ideal for improving the formation by means of revolving dewatering members. A further drawback has been that these prior-art formers have not included the possibility of controlling the proportion of quantities of dewatering taking place through the upper wire and through the lower wire. In several cases, operators of paper machines have expressed the need for such possibility of control to be provided.
In a manner known in the prior art with respect to twin-wire formers of paper machines, different types of so-called forming shoes are used as stationary dewatering elements, such shoes being most commonly composed of several ribs transverse to the direction of running of the wire. In addition to the dewatering effect, these ribs also produce pulsation in the partially formed web passing thereby between the two wires. By the effect of this pulsation, the fibers in the web are displaced relative to one another so that fiber agglomerations or flocks present in the web are decomposed to some extent. In such forming shoes, the guide or glide face placed against the wire is curved, and its structure is usually the same over the entire length of the shoe. It has also been possible to note that such a forming shoe operates optimally only within a quite limited speed range of the paper machine, and as a rule, with only a certain specific paper quality produced.
With twin-wire formers, a considerable proportion of the dewatering takes place within the area of the forming shoe. Thus, the forming shoe has considerable potential possibilities of affecting both the dewatering capacity and the web formation. It has, however, not been realized in the prior art how to make use of these possibilities, except to a limited extent. Moreover, it should be noted that the forming shoe acts as an element controlling the joint run of the wires, thereby affecting the stability of the runs of the wires for its part.
With respect to the prior art related to the present invention, reference is made to the Valmet Finnish Patent Application No. 771364 (filed Apr. 28, 1977), in which a method for the manufacture of multi-layer board and a web forming unit are described. As described therein, a forming shoe is used on the twin-wire part in which a length of the open and closed guide face is arranged to be adjustable by means of filler ribs. Thereby, a slot face on the forming shoe can be converted to a completely solid guide face. This forming shoe is expressly intended for the manufacture of multi-layer board.
Reference is also made to the Valmet Finnish Patent Applications Nos. 820742 (corresponding to U.S. Pat. No. 4,614,566) and 821531, in which a forming-shoe construction is described in the latter reference as consisting of two or more rib face components of different curve radii, and whose length proportions are adjustable relative to each other.
In recent years, modernization of existing fourdrinier machines has become common in which one upper-wire unit has most commonly been placed on the fourdrinier wire part jointly operative therewith, and within the area of which the dewatering can also be made to take place upwardly. Both increasing the dewatering capacity and improving the retention are aims or goals. Such wire parts are called hybrid formers. The wire parts of entirely new paper machines can also be designed of the hybrid type, right from the beginning.
As a rule, an increased dewatering capacity permits an increased running speed of a paper machine, or at least, if the running speed remains unchanged, permits a lowering of the consistency of the pulp fed out of the head box which has, in itself, favorable effects. In certain cases, by means of the upper-wire units noted above, older slow newsprint machines have been modernized into board machines, without increasing the running speed of the machines.